In television systems where only the baseband or TV video is utilized such as in studio equipment and the like where no television carrier is present, satisfactory operation of the system is provided only when the video waveform does not vary in amplitude and is not degradated by the loss of high frequency components, loss of low frequency components, addition of white noise, addition of impulse noise, addition of mains frequency signals, sound-in-syncs, step changes in the DC level and missing lines. As a result of these various degradations, synchronization can become inaccurate and may even be lost.
In U.S. Pat. No. 3,699,256 there is described a circuit for detecting a predetermined level of a composite signal, and in particular the blanking level of a video waveform, comprising a low pass filter means providing a given signal output when the video input is gated at a predetermined level and including feedback means for controlling the amplitude or positioning of the waveform until the level is adjusted relative to gating at the desired valve level. This circuit detects the synchronization pulse despite amplitude variations, loss of high frequency components, loss of low frequency components, addition of white noise, addition of impulse noise, and addition of mains frequency signals.
However, in studio equipment such as most video tape recorders, the video waveform is devoid of lines during a portion of the vertical interval time and the level thereof is at the blanking level. As such, the circuit described in the aforementioned patent is inconsistent with satisfactory operation of video tape recorders (VRT) sources, a definite disadvantage. Also, when an encoded digital signal such as sound-in-syncs is inserted into the composite video waveform (sound-in-syncs is inserted at the horizontal rate during sync tip time), the circuit of the aforementioned patent recognizes the sound-in-sync edges as excessive sync and this, of course, is highly undesirable.